This invention relates to acoustic devices capable of acoustic action involving bending waves.
Co-pending International Patent Application PCT/GB96/02145 (published W097/09842) includes various teaching as to nature, structure and configuration of acoustic panel members having capability to sustain and propagate input vibrational energy through bending waves in operative area(s) extending transversely of thickness usually (if not necessarily) to edges of the member(s). Detail analyses are made of various specific panel member configurations, with or without directional anisotropy of bending stiffness across said area(s), so as to have resonant mode vibration components distributed over said area(s) beneficially for acoustic coupling with ambient air. Analyses extend to predetermined preferential location(s) within said area(s) for transducer means, particularly operationally active or moving part(s) thereof effective in relation to acoustic vibrational activity in said area(s) and signals, usually electrical, corresponding to acoustic content of such vibrational activity. Uses are also envisaged in the above PCT application for such members as or in xe2x80x9cpassivexe2x80x9d acoustic devices, i.e. without transducer means, such as for reverberation or for acoustic filtering or for acoustically xe2x80x9cvoicingxe2x80x9d a space or room. Other xe2x80x9cactivexe2x80x9d acoustic devices, i.e. with bending wave transducer means, include a remarkably wide range of loudspeakers as sources of sound when supplied with input signals to be converted to said sound, and also in such as microphones when exposed to sound to be converted into other signals.
Co-pending International Patent Application PCT/GB98/00621 concerns applying to panel member(s) distribution(s) of stiffness(es) and/or mass(es) not centred coincidentally with centre(s) of mass and/or geometrical centre(s) . This is particularly (but not exclusively) useful to beneficially combining both pistonic acoustic action (as for hitherto conventional, typically cone-type, loudspeakers) with bending wave acoustic action generally as in the above published PCT application. Specifically, location(s) of transducer means for both pistonic and bending wave actions can include at centre(s) of mass and/or geometrical centre(s) (as very much suits pistonic action), but still satisfy general desiderata for bending wave action.
This invention has arisen from intuitive feeling that various approaches of the above PCT applications to design and specification of acoustically useful bending wave action members reflect some other useful concept/methodology that should be capable of yielding as good or yet better and/or as practical or more practical design/specification criteria, perhaps including other useful configurations and transducer locations not before specified or otherwise appreciated. It has been an object of this invention to investigate, and arrive at such results.
According to first general method and device aspects of this invention, panel member parameters affecting bending wave action, such as particularly configuration/geometry in relation to bending stiffness(es) and/or bending wave transducer location(s), is/are in accordance with desiderata applied to analysable characteristic(s) relevant to power transfer for the acoustic device concerned, such desiderata usefully favouring acceptable distribution and/or density and/or evenness of excitation of acoustically relevant resonant modes of surface vibration involved in bending wave action.
It has been particularly established that desirably effective resonant mode density/distribution correlates with a measure of smoothness of power transfer for the acoustic device concerned; and use and results of such correlation in terms of acoustic panel members involving bending wave action constitute various other aspects of this invention.
Underlying inventive rationale or concept involved includes appreciation that, for active acoustic devices as sources of sound, satisfactory acoustic performance of panel members concerned is more dependent on smoothness of power output than on hitherto conventionally esteemed flatness of output over whatever frequency range is concerned/desired. Deviation from flatness of output is actually readily compensated by suitable electronic signal conditioning, specifically so long as the output deviations concerned are reasonably smooth.
Energy losses within panel members and transducer means of acoustic devices concerned tend to be both relatively small and reasonably smooth in themselves. Accordingly, for the purposes hereof, effectiveness of device design and specification can be based on smoothness of input power transfer, including particularly as to geometry/configuration such as aspect ratios and as to bending wave transducer location(s) such as in terms of proportionate co-ordinates.
Whatever particular characteristic(s) is/are involved in assessing smoothness of power transfer, conveniently and preferentially input power transfer, it is practical to be concerned with deviation from some useful condition, state or value, whether of arbitrary or of relational nature. Thus, analysis relative to same or unity weighting of whatever resonant frequency modes are concerned has produced useful results, as has analysis relative to mean value(s) . However, selective adjustment of weighting etc is also seen as useful refinement, for example at least for end-most modal frequencies involved, particularly lowest; and feasibly more generally or otherwise.
The frequency modes concerned/involved in analytical assessment hereof can be as arise from making practically viable simplification, such as using analogies of one-dimensional nature, say to orthogonal beams notionally in directions parallel to pairs of opposite sides of substantially rectangular panel members. This simplification approach reflects success achieved in specific teaching of W097/09842, including first consideration relative to a number of resonant modes in each beam direction and directly related inter-active modes. Refinements of analyses relative to two-dimensional relationships should more closely reflect realities of panel members as such, including revealing and taking appropriate account of more inter-actively related resonant modal frequencies.
Preferred said characteristic(s) relevant to power transfer for the panel member include criteria for mechanical impedance, say as to standard deviation with application of a smoothing factor, say 10%.
In some particular inventive aspects hereof, criteria for mechanical impedance are used in assessing input power transfer, specifically in finding practical geometries and/or stiffness parameters/distributions of panel members for acoustic action relying on distribution of resonant modes of bending wave action. It can be of high practical value first to investigate relative to known favourable transducer locations and to present results functionally, usefully graphically, relative to variant aspect ratios of general geometrical shape concerned in looking for minima of deviation.
In other particular inventive aspects hereof, criteria for mechanical impedance is/are used to find practical transducer locations for particular desired geometries/configurations and/or stiffness distributions of panel members for acoustic action involving bending waves, specifically and advantageously without limitation to panel members having favourable geometry/configuration such as available from said some inventive aspects. It can be of high practical value to investigate variable one relative to fixed other of co-operative areal locators such as co-ordinates of transducer location and present results functionally, usefully graphically, in looking for minimum deviation of preferably smoothed mechanical impedance. It can also be of high practical value to present results of this investigation of panel members as areal distribution of mechanical impedance or deviation thereof, conveniently in contoured manner to indicate extremes and gradations between, and for which it is a matter of choice whether to apply selected values and/or to normalise relative thereto, or to do no more than have relative step-wise gradations indicate at least best and worst locations, say within 10% or less steps.
In further aspects of invention hereof, geometries promising for acoustic action involving bending waves are investigated using a measure of mechanical impedance for promising transducer locations, and such promising geometries are further investigated in relation to use of such promising transducer locations, such investigations being capable of application cumulatively/successively/recursively for any desired degree of further refining of both of promising geometrical parameters and promising transducer location parameters.
For substantially rectangular panel members and methodology, simplification based analyses involving superposition of orthogonal beam-type functions, and with reference to 10% smoothness criteria for mechanical impedance, have confirmed and refined calculation for one known preferred aspect ratio, specifically 1:1.134 as taught in above published PCT application, to be at about 1.138:1; and refined proportionate co-ordinates for transducer location (4/9, 3/7) thereof to about (0.440,0.414). In addition, however, and starting from substantially the same transducer location co-ordinates, analyses hereof have revealed another promising aspect ratio, specifically at about 1.41 to about 1.47. In practice, particular investigation of 1.47 aspect ratio with transducer locations substantially at proportionate co-ordinate position(s) (4/9, 4/9) led by cumulative refinement to aspect ratio 1.41 and transducer co-ordinate locations 0.455, 0.452; indeed, to appreciation that there may be considerable inter-relationship between these 1.41 to 1.47 aspect ratios and variant transducer locations.
It is a particular inventive aspect hereof that a substantially rectangular panel member (as or in an acoustic device and relying on bending wave action) and substantially isotropic as to its bending stiffness in at least two directions has an aspect ratio of about 1.41:1 to about 1.47:1; and another particular aspect of invention that proportionate co-ordinate transducer location(s) involve substantially 0.453 and/or substantially 0.447.
Moreover, two other reasonably promising aspect ratios have also emerged from further development of simplified beam type analyses, namely about 1.6 and about 1.2, together with viable transducer locations at (0.41, 0.44) and (0.403, 0.406), respectively; again with expectation of useful inter-relationships between particular aspect ratios and particular transducer locations.
It has further been established for the purposes of this invention that, perhaps particularly for panel members of favourable geometries/configurations, including such variations as known to arise from anisotropy of bending stiffness(es), the above attainable high specificity as to transducer locations amounts to refined determination within more extended areas that are generally favourable in terms of transducer locations. Indeed, there is strong correlation between size of such areas, particularly medial but off-centre for panel members with isotropy of bending stiffness, and favourability of geometry/configuration, thus between what might be termed truly significant high specificity and unfavourability of geometry/configuration. At least for the latter, it can be particularly valuable to utilise accompanying analyses by scrutiny of power output with frequency and/or finite element analysis (FEA) at least to assess low frequency modality, say as indicative of start positions for analysis of transducer location as above (or below) and/or of overly intrusive resonant modes for useful correction by localised clamping/damping or for compensation by signal conditioning. Interestingly, for favourable substantially rectangular geometry/configuration viable edge adjacent transducer locations are indicated on basis of mechanical impedance characteristics/desiderata.
The above-indicated alternative techniques utilising inherently two-dimensional analysis, also in terms of mechanical impedance, generally confirm efficacy of above aspect ratios and transducer locations, including promising relatively discrete and extended areas, whether or not for hitherto favoured aspect ratios, thus efficacy of such methodology and results with manifest merit of a general nature even including converse approaches identifying particularly poor areas to be avoided for transducer location and/or aspect ratios of low prospects (albeit then capable of indicating possibly or likely viable, or best attainable singly or in combination, transducer locations in unfavourable geometries).
It is of particular practical interest that hitherto known least promising or worst cases of most symmetrical geometries, such as isotropic as to bending stiffness within square or circular boundaries, and substantially central locations of transducers, continue to be indicated as poor combinations, but that much more or most promising transducer locations can now be identified even to the point of viability at least for perhaps relatively limited frequency ranges and output responses.
Inventive methodology hereof and results obtainable can take account of boundary conditions ranging from free or only lightly damped to more strongly damped and constrained including clamped for which promise is, if anything, now highest (and practically highly beneficially so in relation to actual physical implementation and presentation of acoustic devices hereof, particularly in or as panel-form loudspeakers).